Handle movies without a backdrop

If a movie doesn't have a backdrop (like http://www.themoviedb.org/movie/199620-walking-with-dinosaurs), `Movie.get_backdrop()` raised a `TypeError` because `img_path` would be `None`, and you can't concatenate `None` with a string.

Ideally, there needs to be checks for the other `get_*` methods, but this is the one that's bugging me now.